Plasma processing systems have long been employed to process substrates (e.g., wafers or flat panels or LCD panels) to form integrated circuits or other electronic products. Popular plasma processing systems may include capacitively coupled plasma processing systems (CCP) or inductively coupled plasma processing systems (ICP), among others.
Generally speaking, plasma substrate processing involves a balance of ions and radicals (also referred to as neutrals). For example, with a plasma that has more radicals than ions, etching tends to be more chemical and isotropic. With a plasma that has more ions than radicals, etching tends to be more physical and selectivity tends to suffer. In a traditional plasma chamber, ions and radicals tend to be closely coupled. Accordingly, the process window (with respect to processing parameters) tends to be fairly narrow due to the fact that there are limited control knobs to independently achieve an ion-dominant plasma or a radical-dominant plasma.
As electronic devices become smaller and/or more complex, etching requirements such as selectivity, uniformity, high aspect ratio, aspect dependent etching, etc., have increased. While it has been possible to perform etches on the current generation of products by changing certain parameters such as pressure, RF bias, power, etc., the next generation of smaller and/or more sophisticated products demand different etch capabilities. The fact that ions and radicals cannot be more effectively decoupled and independently controlled has limited and in some cases made it impractical to perform some etch processes to manufacture these smaller and/or more sophisticated electronic devices in some plasma processing systems.
In the prior art, attempts have been made to obtain plasma conditions to modulate the ion-to-radical ratio at different times during an etch. In a conventional scheme, the source RF signal may be pulsed (e.g., on and off) in order to obtain a plasma that has the normal ion to neutral flux ratio during one phase of the pulse cycle (e.g., the pulse on phase) and a plasma with lower ion to neutral flux ratio during another phase of the pulse cycle (e.g., during the pulse off phase). It is known that source RF signal may be pulsed synchronously with bias RF signal.
However, it has been observed that while the prior art pulsing has, to some extent, resulted in alternate phases of normal ion to neutral flux ratio plasmas at different points in time and has opened up the operating window for some processes, larger operating windows are still desired.